The vast majority of ink jet printing devices can be reduced to two basic categories:
devices in which a transducer (typically of the piezoelectric or similar type) produces a pressure pulse intended to effect the ejection of at least one drop of ink from a nozzle, and PA1 devices in which thermal energy is used to produce a vapor bubble in a channel or chamber filled with ink so as to effect the ejection of at least one drop of ink. PA1 the problem of producing defect-free semiconductor wafers of large dimensions in a sufficient yield to allow economical components to be obtained, and PA1 the risk of being faced at the end of the manufacturing process with a defective device simply as a result of the fact that a single one of the many ink ejection nozzles and of the relative heating elements (also present in several thousands in a parallel device) is not functional. These problems have hitherto been such as to render the production of devices of this kind relatively unattractive from an economical point of view.
The present invention has been developed with particular attention to possible use in a printing device of this latter type, usually defined as a thermal ink jet printing device.
A detailed description of the basic principles of operation and of numerous possible structures of printing devices of this type is provided in the document U.S. Pat. No. 4,463,359.
Devices of this kind form the subject of rather intense patent literature, as demonstrated by way of example by the documents U.S. Pat. No. 4,985,710 and U.S. Pat. No. 5,160,945, as well as in the other patent documents cited therein.
Thermal ink jet printing devices are usually grouped in turn in two broad categories, designated "roofshooter" and "edgeshooter" respectively, as a function of the particular method for the production of the ink ejection nozzles. As will be immediately clear to the person skilled in the art, the detailed description provided hereinafter by way of example relates to a device of the "roofshooter" type. It will nevertheless be clear that the invention is not limited to such a specific construction, but can be used for the production of devices with a construction of the "edgeshooter" or other type.
Thermal ink jet printing devices are usually manufactured using semiconductor wafers and processing technology typical of the production of integrated and/or hybrid circuits. This allows, inter alia, for the production of multiple heating elements (resistors) with extremely small dimensions with associated relative control circuits (for excitation of the heating resistors) and the relative hydraulic system for supplying the ink.
This solution is ideal for the production of printing devices (heads) of small dimensions which can be associated with a cartridge containing a store of ink and can be mounted on a carriage which, during use, is moved transversely with respect to the surface to be printed, all in accordance with a typical serial printing method.
In practice, once the head has been displaced transversely with respect to the printing surface in order to print a line or strip (referred to as a "swath"), the printing surface is advanced by a corresponding amount and a transverse movement is thus imparted once again to the head in order to print another line or strip.
This same technology is also suitable, at least in principle, for the production of parallel printing devices (usually defined as the "pagewidth" type) capable of printing a line or strip in one single stroke, i.e. without requiring any scanning movement across the surface just printed.
It will be clear in this connection that the potential field of application of the printing devices in question is not limited to the traditional areas of information and office technology (printers for processing systems, typewriters, photocopiers, facsimile machines, etc.), but, above all, as a result of the fact that it can be extended to colour printers, embraces many different fields, such as the printing of textiles and decorative sheets in general. This latter field of application is very broad and promising, particularly as a result of the possibility offered by ink jet devices of avoiding the need for recourse to relatively inflexible and uneconomical traditional printing processes (with the preparation of formes, etc.) when it is desired to produce very small batches of similar products (e.g. to produce textile samples or the like).
As is well illustrated in U.S. Pat. No. 4,985,710 already mentioned hereinbefore (see in particular column 2, lines 40 to 56), the possibility of producing thermal ink jet printing devices operating with a parallel format meets with various difficulties of a technological nature, e.g.:
Precisely in order to overcome this problem, it has already been proposed to assemble together several elementary modules of smaller dimensions in order to produce parallel devices. It may be useful to refer in this connection to FIG. 17 of U.S. Pat. No. 4,463,359 and to the relative description, as well as to the entire description and drawings of U.S. Pat. No. 4,985,710 and U.S. Pat. No. 5,160,945, all already mentioned hereinbefore.
However, other problems arise when adopting a modular structure, e.g. that of obtaining exact alignment of the ink ejection nozzles of the various modules, or that of ensuring that the modules assembled to form a parallel device are actually functional.
To this end, U.S. Pat. No. 5,160,945 proposes producing the parallel structure from modules (or subunits) which are defined as "fully functional". Although this concept is not defined more specifically, the description and the drawings of U.S. Pat. No. 5,160,945 relate to a structure in which a plurality of printing subunits (preferably of the roofshooter type) are mounted on the surface of one side of a structural base plate or support. A passageway is formed in the plate and, adjacent to the lateral surface containing the printing units, openings are provided between the said passageway and the ink inlets of the individual printing units mounted on the plate in such a manner that the ink supplied to the passageway in the plate is distributed to the various printing units.
Precisely as a function of this structure, the solution according to U.S. Pat. No. 5,160,945 does not completely solve the problem of the reliability of the printing device in its entirety with respect to the hydraulic supply and the ink ejection.